The invention relates to a method and an apparatus for controlling the fuel pressure in a common rail fuel injection system, wherein a rail pressure deviation is determined by a comparison of the desired and the actual rail pressure and wherein a rail pressure control value for controlling a throttle valve by way of a rail pressure controller is calculated from the rail pressure control deviation and the fuel supply to a high pressure pump and, consequently, the rail pressure is controlled.
In a common rail fuel injection system, the fuel is pumped by a low-pressure pump from the fuel tank to a high-pressure pump. The high-pressure pump supplies the fuel with an increased pressure to a rail (high pressure storage). In the flow path between the low pressure pump and the high pressure pump, there is a controllable suction throttle valve by way of which the fuel admission to the high pressure pump is controlled.
DE 103 30 466 B3 discloses such a common rail system wherein the rail pressure is controlled by an electronic control unit disposed in a rail pressure control circuit providing a control value corresponding to the rail pressure. By a filter arranged in a feedback branch, noise signals are suppressed such as signals which have the same frequency as the injection frequency or the pumping frequency of the high pressure pump. The filtered rail pressure signal is compared as rail pressure actual value with a desired rail pressure value resulting in a rail pressure control deviation. From the rail pressure control deviation, the rail pressure controller determines a control value, that is a desired volume flow. This control value is then converted to a pulse-width modulated signal (PWM). This signal is applied to the suction throttle valve for controlling the rail pressure.
The ohmic resistance of the suction throttle valve winding however changes with the temperature. This means that the rail pressure controller calculates different control values for the same stationary operating point, for example, different integration components. During stationary engine operation, the integration component of the rail pressure controller is additionally deposited in a leakage performance graph. Upon failure of the rail pressure sensor then, instead of the control value computed by the rail pressure controller, a value from the leakage performance graph is used. However, this may be problematic as the quality the rail pressure control may then suffer upon failure of the pressure sensor.
A measure for decreasing the temperature dependency of a rail pressure control circuit is known from DE 198 02 583 A1. Here, the rail pressure controller is provided with a current control circuit. The guide value of the current control circuit corresponds to a desired electric current, which is provided by the rail pressure controller as a control value. By way of a current sensor, the electric current which flows through the winding of a pressure control valve is determined from the actual current value. From the control deviation between the desired current value and the actual current value the current controller determines a control value. The current control of the pressure valve is absolutely necessary since the pressure control valve is arranged at the high pressure side and controls the fuel release from the rail back to the fuel tank. Since a pressure of about up to 180 bar is present in the rail, during a throttling control to a pressure of 0 bar, a large amount of heat is released. From this as well as by the application of the electric current the temperature of the winding is increased. With the control circuit shown, a pulse width modulated signal is applied to current controller as input signal. Since a current controller must be highly dynamic the application of an unfiltered PWM signal may result in an instability of the current control circuit. There is no backup for the current control circuit in case of a current measurement error or failure.
It is therefore the object of the present invention to provide a stable and temperature-independent rail pressure control circuit with a suction throttle valve which additionally includes an error protection.